Analysis of gas mixtures and determination of the partial pressures of individual gases is gaining increasing significance. Accordingly gas sensors for automatic control in process sequences, for ecological monitoring and in biotechnology already are known. As a rule gas sensors make use of solid ionic conductors which offer many advantages compared to liquid electrolytes. A distinction is made regarding the sensors known to date between the potentiometric and the amperometric ones, the latter sometimes also being called polarographic sensors.
The earliest known gas sensors are potentiometric concentration cells with a gas electrode. The emf in them is according to Nernst's equation, ##EQU1## where R is the gas constant, F the Faraday constant, T the absolute temperature and n the number of electrons taking part in the electrode reaction.
The first such cell that was used as a gas sensor contains zirconium oxide as the solid electrolyte and is composed as follows EQU Pt, O.sub.2 [pO.sub.2 (ref)]/ZrO.sub.2 (CaO)/O.sub.2 [pO.sub.2 ], Pt.
If the partial pressure of the reference oxygen, i.e. its activity is kept constant in this early system, then the unknown partial pressure pO.sub.2 can be measured by means of the voltage E.
Such zirconium-oxide electrolyte based oxygen sensors have been commercially available since about 1965. Moreover gas sensors also are now known which allow determining other gases such as Cl.sub.2 and SO.sub.2 /SO.sub.3 by means of solid electrolytes [J. Fouletier, Sensors and Actuators, 3, 1982-3, 295; and W. L. Worrell et al, Sensors and Actuators, 2, 1982, 385].
Potentiometric gas sensors already are known, in which the solid electrolyte evinces a modified surface layer. Such sensors are a voltaic cell composed as follows: EQU Reference-electrode/solid-electrolyte/modified surface-layer/electrical-conductor/gas.
In these sensors, the equilibrium between electrolyte and gas is set by additional intermediate layers, the so-called gas-sensitive layers. Conventionally a film or a thin layer together with the electrically conducting material is deposited on the electrolyte (German patent document A 2,926,172). A series of sensors based on this principle already is known, allowing to determine Cl.sub.2, NO.sub.2, O.sub.2 and CO.sub.2 (W. Weppner et al, Solid State Ionics, 18+19, 1986, 1223 and Weppner et al, Sensors and Actuators, 12, 1987, 449 and J. Liu and W. Weppner, Solid State Communications 76, 1990, 311).
Contrary to the above described potentiometric sensors, amperometric solid-electrolyte gas sensors are based on the principle of the saturation current. Amperometric sensors are significantly more sensitive to comparatively slight changes in pressure and therefore reveal the partial gas pressure with higher accuracy. Because the signal, namely the saturation current I.sub.lim is directly proportional to the partial gas pressure of the gas being measured, the measuring apparatus is much simplified. Moreover, the saturation current is comparatively insensitive to the system temperature and total pressure. Again amperometric sensors may be operated at substantially lower temperatures, that is at temperatures less by 200.degree. C. than potentiometric sensors.
Amperometric sensors merely require a voltage high enough to drive the sensor in the region of the saturation current. Lastly, and contrary to the case for potentiometric gas sensors, amperometric gas sensors also can be operated without reference electrodes, as a result of which both design and operation are much simplified.
Such amperometric oxygen sensors already are known from Dietz in solid State, Ionics 6, 1982, 175 and from H. Jahnke et al, Ber. Bunsenges. Phys. Chem. 92, 1988, 1250 and from Takeuchi et al, Chemical Sensor Technology vol. 1, T Seuyama ed., Elsevia, Amsterdam, 1988, p 79. Cubic stabilized zirconium oxide (CSZ) is used in the sensors described therein.